Natural testosterone in the body is a beautiful thing. It is produced by the testes upon orders form the pituitary gland, and all is controlled by the endocrine system. It is delivered consistently to the body and its delivery is regulated based upon need. When functioning perfectly, it's a well-oiled machine that does collect some rust as it ages, and there is little issue with prostate health until age 40.
Injected testosterone is a whole another story. Converted to DHT (dihydrotestosterone) by the body, it immediately sets off a chain of events, which can be hard for the body to overcome. The pituitary gland orders the testes to halt production of testosterone once it senses the presence of high artificial levels of testosterone or DHT in the body. The receptors in the prostate react to the DHT by creating growth of the prostate, which can cause trouble with urination and lead to growth of cancer cells. The receptors in the liver react to the presence of DHT by spiking enzyme levels and fostering an environment ripe for tumors. Additionally, the receptors in the scalp sense the presence of DHT and accelerate baldness patterns, which would have been destined for far later in life.
Most of these side effects are reversible, and shouldn't deter most people from considering the benefits of anabolic steroids. For users under 40, the prostate returns to regular size shortly after DHT levels drop. The liver enzymes return to normal levels, and any small polyps or tumors quickly dissipate. The male pattern baldness, however, is a permanent thing. Once you enter the world of DHT, you choose permanent risks of hair loss. Medications such as Propecia help to reduce the scalp's responsiveness to DHT.
Once you realize that steroids aren't the problem, and testosterone isn't the problem - but that the problem lies in the unnatural presence of DHT in the body, you can plan cycles, additional medications, and long-term goals accordingly. Use steroids after age twenty to ensure your testosterone levels have already began to drop. Use steroids before age 40 to ensure you don't risk unnecessary prostate growth. Remember that the receptors will be most responsive to the presence of DHT the first time you run a cycle. After that, the body will respond, but never with the same voracity. For this reason, the first steroid cycle should be highly planned, very effective, and with a wary eye upon the side effects, as the prostate, liver, and scalp receptors will be most responsive to them during the first cycle as well.